The long-term goal of this research program is to investigate how a cell gauges the status of DNA replication to maintain the temporal order of the cell cycle. In a normal cell cycle, entry into mitosis is dependent upon the completion of S phase and this dependency is maintained by a control mechanism termed checkpoint control. Genetic studies of yeast have suggested that once cells exit G1 and pass START, cells are committed to enter both S phase and mitosis. The presence of a replication complex signals the cell that it is in S phase and restrains the cell from entering mitosis prematurely; subsequent disassembly of the replication complex then activates the onset of mitosis. We propose to use fission yeast as a model organism and DNA polymerases alpha and delta and PCNA as probes to investigate what gene products generate a signal or sense and transmit a signal to restrain cells from entering mitosis when initiation or replication is faltered due to defects in one of these replication enzymes or protein. The specific aims are: (I) Characterize a panel of mutants that fail to restrain cells from premature mitotic entry when S-phase progression is delayed due to a semi-disabled DNA polymerase delta. (II) Identify genes that detect the initiation defects caused by a semi-disabled DNA polymerase alpha to prevent cells from premature mitotic entry. (III) Identify alleles of PCNA that are involved in S phase progression, and isolate and characterize gene products that interact with these alleles. These studies will provide a foundation for further understanding of cell cycle checkpoint mechanisms. Knowledge gained from the fission yeast studies can be applied to human cells in the future, and thus will contribute to the basic understanding of the molecular etiology of neoplasia.